Until now, as anti-corrosive pigments, there have widely been used lead-containing anti-corrosive pigments such as red lead, lead cyanamide and lead suboxide; and chromate-containing anti-corrosive pigments such as zinc chromate and strontium chromate.
These lead-containing and chromate-containing anti-corrosive pigments have excellent anti-corrosive characteristics, but the use thereof has gradually been regulated while taking into consideration, for instance, safety, hygiene, and environmental pollution. More specifically, it is apprehended that they may hurt the human health.
On the other hand, there have likewise been proposed phosphoric acid salt-containing pigments, molybdate-containing pigments, organic phosphonic acid salt-containing pigments and borate-containing pigments as environmentally pollution-free anti-corrosive pigments.
These environmentally pollution-free anti-corrosive pigments are much less problem concerning the safety and hygiene, but the anti-corrosive properties thereof are in general inferior to those observed for the lead-containing and chromate-containing anti-corrosive pigments. Furthermore, these environmentally pollution-free pigments cause various problems, for example, in that they require a high production cost and further they are deteriorated in their physical properties, dispersibility and storage stability.
These environmentally pollution-free pigments in general comprise, as the principal components, solid acids easily dissolved out and accordingly, they likewise comprise, as secondary components, solid base components in order to control any excess dissolution of such acidic components and to maintain the pH value of the resulting coated film at the neutral level. For this reason, most of the environmentally pollution-free pigments currently put on the market comprise zinc oxide as such a solid base component.
Nevertheless, it has been considered questionable whether zinc-containing compounds, in particular, zinc oxide used as a pigment component is safe or not mainly in the United States and Europe, along with the recent intensified regulation of the environmental pollution. This is because, it has been proved that zinc oxide may adversely affect the oceanic life or the marine animals and accordingly, there have been such a tendency to start self-controlling of the use of zinc oxide in the coating or painting of, for instance, containers for transporting foods, not to mention the use thereof in the places where it may have a bad influence.
In addition, when incorporating such a zinc oxide-containing anti-corrosive pigment into an oily vehicle such as boiled oil or an oily varnish, the zinc oxide reacts with the vehicle to thus cause a phenomenon or cohesion of pigment components called “seeding” and this in turn becomes a cause of a problem such as the development of insufficient appearance of the resulting coated film and the reduction of the physical properties of the same, as has been described in JIS K5621. Moreover, the coated film containing such a zinc oxide-containing anti-corrosive pigment causes a problem in that it is liable to develop white rust on a zinc-plated plate.
Thus, there have been developed environmentally pollution-free, anti-corrosive pigments, which do not contain zinc oxide at all (see, for instance, Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) Nos. Hei 8-283619, Hei 11-049979 and 2003-113482).
However, these environmentally pollution-free, anti-corrosive pigments are still insufficient in both anti-corrosive properties and wide-spread applicability.
For instance, J.P. KOKAI 2003-113482 proposes a method for preparing an anti-corrosive pigment composition starting from precipitates obtained through a reaction of sodium tripolyphosphate with a water-soluble alkaline earth metal compound under wet conditions. However, the anti-corrosive pigment composition thus prepared never ensures the desired long-lasting anti-corrosive properties since it comprises a large amount of water-soluble ions such as Na+, NO3− and Cl−.